Elastomeric gaskets and seals are generally prepared by processes that are based on compression molding, transfer molding or injection molding techniques wherein a polymer, fully compounded with curing agent and other additives, is introduced into a mold that is then heated under pressure. The elevated temperatures used during the molding process cause chemical reaction of the elastomer with the curative, thereby producing a crosslinked elastomer. As a result of the chemical reactions that occur between the polymer and curative, a network structure is generated throughout the cured, i.e. crosslinked, product. This network structure confers the strength and elasticity that give elastomeric products their characteristic ability to seal under compression.
Normally, elastomeric seals and gaskets produced by any of the molding techniques mentioned above are fitted manually onto an article to be sealed. Alternatively, adhesives are sometimes utilized to attach the cured sealing member to an article. In addition, gaskets may be molded on a metal carrier, for example in the manufacture of shaft seals. Such attachment techniques are not completely satisfactory in all cases. In particular, manual methods are time consuming, adhesives can affect the physical properties of the seal, and molding onto metal carriers requires long cycle times and high temperatures.
Finished seals are often used in grooved articles as sealing means, for example in rocker covers and air intake manifolds that are used in automobile engines, and in bonded door assemblies that are used in electronic and processing equipment. Such grooved parts often have complex cross sections that aid in maintaining the seal in place in the groove during dynamic applications. Production of seals for such grooves involves design of correspondingly complex molds. In addition, insertion of the elastomeric seal into a groove represents a separate, manual step in fabrication of a finished part that can be time-consuming. Alternatives, such as cure in place processes, are not viable in some instances.
Cure in place gasketing techniques that depend on utilization of ultraviolet (UV) radiation, rather than thermally initiated chemistry are known. For example, U.S. Pat. No. 6,346,300; 6,472,452; 6,495,213; and 6,506,460 describe processes that can be used with a variety of elastomers, such as fluoroelastomers, ethylene (meth)acrylate elastomers, ethylene alpha-olefin elastomers, and chlorinated elastomers. The use of silicone elastomers in similar processes is disclosed, for example, in B. G. McClelland, Cure-In-Place Gasketing Offers Process And Quality Improvements As A Water Pump Flange Seal in Gaskets and Sealants For Automotive Applications, SAE Special Publications, 1235, 47-51, 1997. Although these processes yield excellent results, in some instances the technique is not appropriate, in particular when high precision and more complex seal shapes are desired than can be achieved when using dispensed cure in place gasketing techniques.
Injection molding, a technique for processing polymers that is noted for production of parts of uniform and complex cross section, is used for thermoplastic compositions because they flow easily on application of heat and do not require a chemical crosslinking process. Injection molding processes are also used for preparation of thermoplastic segmented elastomer compositions wherein a semi-crystalline composition that is shaped in the molding process will maintain its shape after cooling by virtue of crystallization of its hard segments. Such compositions are easily demolded. An example of a process wherein thermoplastic elastomers are employed is disclosed in U.S. Pat. No. 5,618,488 which describes a method of manufacturing a seal for a roller bearing. However, the technique is not economical when used with thermoset elastomers due to the long processing required to crosslink the elastomer and the inevitable non-recyclable scrap that is formed. Another reason that UV cure processes have not generally employed injection molding machines is that most apparatus and molds are not transparent to UV light and therefore a translucent mold is necessary. U.S. Pat. No. 5,885,514 describes a process wherein articles formed from elastomers that are curable by action of UV light are produced in an injection molding machine having a mold that is transparent to UV radiation. This technique is useful, but specialized equipment is necessary.
Because of the uniform mold filling capability of the injection molding process and the benefits associated with manufacture of gaskets using cured in place techniques, it would be useful to have available an injection molding process that can be employed with conventional equipment to produce shaped curable sealing compositions adhered to a support that could be cured in place to produce articles where the physical properties of the cured sealing compositions are typical of crosslinked thermoset elastomers, but wherein the process has a short cycle time and manufacture does not involve production of substantial scrap.